The present invention relates to an automatic microphone control suited for use in sound reinforcement, recording, broadcast, teleconference and other applications.
Because of the number of participants involved or the number of locations needing sound pickup, multiple microphones are often used in applications such as churches, courtrooms, broadcasting studios, legislative chambers, and conference rooms, and in particular, in teleconferencing. The outputs of these microphones are usually combined in a mixer to feed a sound reinforcement system, a recording device, or a transmission link to a remote location. If a conventional mixer is used with multiple microphones, the room noise and reverberation pickup is increased as compared to a single microphone; also, the tendency for howlback is increased, even though typically only one or two microphones are receiving useful acoustic input (speech) at any given time. This is discussed extensively in "Direction-Sensitive Gating: A New Approach to Automatic Mixing" by Stephen Julstrom and Thomas Tichy, JAES, Vol. 32, Nos. 7/8, 1984, July/August.
Automatic mixers have more recently been employed to "gate" ON (pass to the mixer output) only signals from microphones receiving useful acoustic input. The relative effectiveness of these mixers is primarily a function of the means used to decide when a microphone should be gated ON. A microphone should gate ON quickly and independently in response to valid speech input over a wide dynamic range. Yet it should not respond to background room noise, nor to talkers who would be better picked up by another microphone. Additionally, for proper operation of many teleconference systems, including the system of the aforementioned related application, the room loudspeaker providing audio from the remote location should not trigger microphone gating.
A gating method dependent on a representation of the microphone output level exceeding a fixed threshold level satisfies none of these criteria. Various prior art references suggest different gating methods.
For example, in Dugan (U.S. Pat. No. 3,814,856), the threshold level for each microphone's gating tracks an estimate of the background room noise made from a distant sample taken from a separate noise-sampling microphone or from the average of all microphones in the system. This is done to improve the gating sensitivity while avoiding response to room noise.
In Breeden (U.S. Pat. No. 3,751,602), a simple microphone is, in essence, gated for use in a speakerphone (a simple teleconference system). Here, the noise reference is taken from the single microphone with its level representation processed through a very slow attack, rapid decay circuit. Additional circuitry inhibits microphone gating for loudspeaker sound in most room acoustic environments.
In Maston (U.S. Pat. No. 3,755,625), one and only one of a plurality of microphones is gated ON at any time. To gate ON (and thus gate OFF the already ON microphone), a microphone's level must exceed a fixed threshold and exceed the level of the already ON microphone by a preselected amount, such as 3 dB.
In Kahn (U.S. Pat. No. 4,099,025), to prevent gating a plurality of microphones for a single source, during the time when a microphone's level exceeds a threshold, triggering of all other microphones is prevented for the duration plus a short additional time roughly corresponding to the transit time for the sound to travel to the farthest microphone in the system.
In Schrader (U.S. Pat. No. 4,090,032), a preselectable, fixed threshold is overridden as soon as at least one microphone's level exceeds it and is gated ON. The threshold then varies between a high maximum level and approximately the level of the gated ON microphone with the highest level. Gating ON of more than a few microphones simultaneously, even for multiple sound sources, is strongly inhibited.
In Anderson et al. (U.S. Pat. No. 4,489,442, owned by the same entity as the present application), each "microphone" actually consists of an array of typically two unidirectional microphones mounted back-to-back in a common housing whose output levels are compared. When the level of the "front" microphone exceeds the level of the "rear" microphone by a predetermined amount, typically 9.54 dB (indicating the sound source is within an "acceptance angle"), gating is triggered for the front microphone's signal.
This Anderson arrangement (which also forms part of the preferred embodiment of the related Julstrom application previously referenced), has an effective gating threshold which inherently tracks at about 5 dB above the room noise level at the microphone's location. The Anderson arrangement results in direction-sensitive gating which limits the number of microphones which gate ON for individual sound sources while not causing the gating of any microphone to inhibit the desired gating of any other microphone for other sound sources. Also, the Anderson arrangement allows positioning a teleconference system loudspeaker in such a way that it will not trigger gating of any microphone and will not significantly inhibit desired gating for local talkers.
However, the operating principle of Anderson requires some care in microphone and loudspeaker placement. Anderson also allows a single sound source to trigger gating in a plurality of microphones with overlapping acceptance angles. Anderson requires typically two high-quality matched transducer elements for each "microphone" even though only one of the pair is ever heard. Most significantly, Anderson can have proper gating inhibited by acoustically reflective objects close to the rear of the microphone or by placement of the microphone too far away from the sound source in relation to the reverberant field of the room, thus preventing the microphone from accurately assessing the direction of the sound source.
The sound which a microphone "hears" in a room can be simply described as consisting of two parts: a direct sound which decreases in level 6 dB each time the distance from the source is doubled; and a reverberant field, coming from all directions, which stays substantially uniform in level throughout the room as it decays away.
The direction-sensitive gating technique works well unless the microphone is so far from the sound source that the reverberant field dominates in the microphone's sound pickup. In larger rooms, this will not occur until the microphone is five feet or more away from a talker. However, at this distance, its pickup would be hollow, "barrelly" and perhaps unintelligible. In smaller rooms, such as offices and many conference rooms, the reverberant field may dominate at distances of two feet or less, preventing proper gating using the direction-sensitive microphone technique at convenient talker-to-microphone distances. However, in contrast to the larger rooms, in many of these smaller rooms, the sound pickup quality, even if predominantly reverberant, is still intelligible and subjectively acceptable due to the quick reverberant decay time. The microphones would be usable if they gated properly.
None of the prior art disclosed in the above cited patents fully addresses the goal of attaining maximum gating sensitivity in the presence of varying background room noise, preventing a plurality of microphones from gating ON for a single talker, allowing with little mutual inhibition a plurality of talkers to simultaneously gate ON a plurality of microphones; and doing all this even when operating in a near totally reverberant (i.e., small room) acoustic environment. Additionally, sound from a teleconferencing system loudspeaker must not gate microphones ON, yet desired gating for simultaneous local speech should not be significantly inhibited, again in a very reverberant environment.
It is therefore an object of the present invention to provide an automatic microphone gating method which maximizes sensitivity to speech while avoiding sensitivity to varying background room noises.
It is yet another object of the present invention to allow only the single most appropriate microphone in a system to gate ON for a single talker.
It is another object of the present invention to allow a plurality of talkers to simultaneously gate ON a plurality of microphones, with minimal mutual inhibition of gating.
It is another object of the present invention to prevent teleconference system loudspeaker sound from gating microphones ON, with minimal inhibition of desired microphone gating for locally originating speech.
It is another object of the present invention to enable all the other objects to be met even in near totally reverberant, smaller room acoustic environments.
It is another object of the present invention to allow the creation of "dead zones" in a room where sound sources do not trigger any microphone gating.
It is another object of the present invention to provide a variation whereby microphone gating information can be used to control other functions, such as automatic video camera switching.
It is another object of the present invention to link a gating method described herein with an automatic gain adjusting means to maintain constant reverberant field pickup as the number of widely spaced microphones gated ON varies above zero.
It is another object of the present invention to provide a variation whereby an automatic gain adjusting means maintains constant reverberant field pickup as the number of very closely spaced directional microphones gated ON varies above zero.
It is another object of the present invention to use a gating method described herein in the teleconference system of the previously referenced related Julstrom application yielding the benefits described therein.
It is still another object of the present invention to employ such a teleconference system in a combined loudspeaker-microphone arrangement which optimally exploits the characteristics of the gating method, is easy and foolproof to set up, provides improved sound pickup and production through optimized acoustical and electrical design, works in almost any acoustical environment, and is easily expandable.